Methods of injection molding and heat press forming are used for producing parts with a predetermined shape from a thermosetting resin such as epoxy resin, phenol resin or the like. In the method of injection molding, a resin slurry is poured into a mold with a predetermined shape and then thermally hardened in it. This is widely used as the most commonly molding method for the resins.
A resin composite is formed by mixing synthetic resins with various kinds of inorganic powders, improving hardness and strength rather than a molded body from only resins (see Japanese Patent Publication No. 57-151308 A, 1982). Generally, the resin composite can also be produced by injection molding or heat compression molding method in the same manner as for the resin above.
In injection molding and heat compression molding, however, the resin should be heated in a mold for molding, so that the mold is heated at high temperature. As a result, the thermal deformation of the mold has made it difficult to improve the dimensional accuracy of the resin composite.
Further, the mold used in these production methods must make use of materials with less thermal deformation and must have a shape with a gate and a slag gate for molten starting materials, thus such a special mold has been required. For this reason, use of these production methods has the problem of cumbersome procedures in producing various products in small amounts each, thus causing the production costs to be high.
Furthermore, heating and hardening carried out simultaneously with molding in a mold in one cycle in the above production method make it difficult to reduce the time required for producing products.
In addition, in the case of injection molding of thermosetting resin, resin left in a liner through which molten resin is introduced into the mold should be discarded because it cannot be reused or reproduced. Therefore, a high proportion of starting materials should be discarded in the case of a molded body with relatively small dimension, resulting in low yield. Hence, the cost of the starting materials is rendered high relative to the production cost, and the cost per product is therefore high.
In the case of the most common injection molding, surface defects having uneven portions are generated on the surface of the resulting molded body during injection molding. Examples of such defects are concave weld lines, projected lines or burrs occurring along the connecting portion at which separable half-molds are attached, tapping defects caused by knock-out pins occurring upon release of a product from the mold, and adherent portions of the resin left in a gate formed in a inlet of the mold. Such surface defects worsen qualities in outward appearance and cause a reduction in yield of the products.
The surface of the molded body formed by conventional injection molding or heat compression molding method is shaped by a mold surface, so that the surface of the body is finished to be smooth. For certain utilities, the surface is so smooth that it is necessary to prevent reduction of the bonding strength of solder applied onto the surface or separation of a printed film with characters or patterns from the surface.
In obtaining a porous molded body by conventional injection molding or heating compression molding, it is further difficult to secure and distribute a predetermined amount of pores uniformly inside of the molded body. That is, if the thermosetting resin is a condensation type resin, a hardening agent should be added to the resin in order to initiate hardening of the resin by chemical reaction. For example, if phenol novolak resin is used as thermosetting resin, a hardening agent such as hexamethylene tetramine is used, and hardening occurs by condensation reaction in which hexamethylene tetramine is decomposed by heating to form aldehyde which is then attacks a hydroxyl group in the phenol to lead to cross-linking. The hexamethylene tetramine is decomposed to generate carbon dioxide, ammonia or gases of water etc. along with aldehyde. And in injection molding, these gases in the condensation reaction process prevent pressure transfer required for molding, thus causing moldability to be lowered. For this reason, processing such as breathing is usually carried out during molding, and as a result, pores are hardly observed in the inside and on the outer face of the resin composite.
The porous composite is useful for a sliding member; however, the problem with use of such resin composite with less pores as sliding member is that it is badly worn due to frictional heat caused by sliding with the opposite member, because the thermal conductivity of thermosetting resin present in the sliding face is not high.
To improve the durability and longevity of the sliding members, therefore, it is necessary to apply a lubricant etc. onto the sliding face and to further supply it during use.
On the other hand, in the molding method according to conventional injection molding, the amount of fillers incorporated is limited to about 20 to 30 vol-% and there was a limit to its further increase. That is, the incorporation of a larger amount of fillers raises the viscosity of molten resin to worsen flow characteristics. Residual stress thus results, and the dimensional accuracy of the molded body is deteriorated, and defects such as cracks, warp, etc. are generated.
As described above, use of injection molding and heat compression molding has a limit to the increasing of the amount of fillers incorporated into the composite, and therefore, it is also difficult to improve the thermal resistance, tracking resistance of the composite. In use of the molded body according to the conventional molding method, there is a limit to its application to parts for safety devices etc. requiring characteristics such as high resistance to heat and tracking.